The XQueryDeviceState
request queries the state of an input device. The current state of keys
and buttons (up or down), and valuators (current value) on the device
is reported by this request. Each key or button is represented by a bit
in the XDeviceState structure that is returned. Valuators on the device
report 0 if they are reporting relative information, and the current value
if they are reporting absolute information.

An invalid device was specified. The specified device
does not exist or has not been opened by this client via XOpenInputDevice.
This error may also occur if some other client has caused the specified
device to become the X keyboard or X pointer device via the XChangeKeyboardDevice
or XChangePointerDevice requests.